This invention relates to a method for improving the hot workability of an age hardenable nickel base alloy and to an alloy having such improved hot workability properties.
In the commercial production of certain age hardenable nickel base alloys, severe difficulties have been encountered during hot rolling of the cast ingots and wrought billet, resulting in cracking along the surface. This cracking necessitates significant amounts of grinding and loss of usable alloy, thereby significantly lowering the yield. Problems with hot working have also been experienced during subsequent forging of the wrought bar into parts or shapes, resulting in cracking.
One such alloy is commercially known by the designation U-720 and has the following nominal composition: about 18 percent chromium, about 5 percent titanium, about 2.5 percent aluminum, about 14.75 percent cobalt, about 3 percent molybdenum, about 1.25 percent tungsten, about 0.035 percent boron, about 0.035 percent carbon, about 0.037 percent zirconium, up to 0.1 percent columbium, up to 0.1 percent tantalum, up to 0.1 percent vanadium, up to 0.1 percent copper, up to 0.50 percent iron, up to 0.15 percent silicon, up to 0.15 percent manganese, up to 0.1 percent phosphorus, up to 0.0025 percent silver, up to 0.01 percent sulfur, and the balance nickel.
The above-noted copending application has disclosed that significant improvements in the hot workability of certain age hardenable nickel base alloys, such as U-720, can be achieved by deliberate additions of lime and magnesium under specified conditions during melting of the alloy. More specifically, significant improvements in the hot workability of the alloy are achieved by melting the appropriate raw materials for the alloy under a vacuum in the presence of lime and forming a desulfurizing lime slag on the surface of the molten raw materials, and thereafter adding a small but significant amount of magnesium thereto just prior to casting the alloy, preferably while under an inert gas atmosphere.